1. Technical Field
The present invention relates to the field of plasma processing and, more, particularly to the method and system of supplying the energy for use in the plasma apparatus.
2. Background Art
Plasma processing equipment is used extensively in the industry for the modification of materials. These modifications include etching, deposition, and ion implantation for fabrication of microelectronic circuits in semiconductor devices. As technology advances due to competition, industry requires and benefits from improving control over the properties of these plasmas.
A plasma is an unusually high-temperature gas that is so highly ionized it is electrically conductive and susceptible to magnetic fields. A variety of techniques are known for energizing the gas. One commonly used technique is the energizing of the gas using a high-frequency power supply. A common practice is to use high frequency alternating current (AC) fields to energize or excite the gas. Techniques for utilizing radio frequency (RF), as well as microwave fields of higher frequencies are well-known in the art.
In a general plasma apparatus, an electrode is disposed in a reaction chamber and the high-frequency power supply is connected to the electrode through a matching network and a feeding cable. The matching network consists of a variety of interfaces and control circuits. The object of most plasma process tools is to match the impedance of the cable and the plasma chamber electrode so that reflectance loss is avoided and most of the power is sent to the plasma. (This will be referred to as the "normal case.") Reflectance loss is the loss caused by the reflection of an electromagnetic field at a discontinuity in a transmission line. A nonuniform filter will transmit a wave undisturbed provided that the iterative impedance of each filter section is made equal. If the iterative impedances of the sections are different, loss of power output occurs due to reflections of the wave occurring at the junctions. As previously stated, in most systems the process engineer's goal is to match the impedances of the sections of a system to ensure optimum conditions for transfer of power from one part of the system to another. Reflections of wave power in a transmission line are eliminated by making the plasma load impedance equal to the generator output impedance, and the line impedance of the transmission line equal to both of the above impedances.
Another reason process engineers do not like mismatches in the impedances between the sections is that it means that the process engineer is not able to tell the difference between the reflected power due to mismatches from the reflected power which indicates a problem in the tool. The only difference is that the reflected power indicating a tool problem would have a different phase. Therefore, the goal of most process engineers is to try and obtain the closest matching possible in the network.
A paper by Masaki Shinomiya, "The Matching Network of the Inductively Coupled Discharge Lamps", J. Light & Vis. Env., Vol. 19, No. 2 (1995), p. 33, and hereby incorporated by reference, shows how to design a stable matching network for a voltage source.